But Chris Grayling, his Tory shadow, said: "If he gets severely criticised by the standards commissioner I think there will be real doubts about his future."

Mr Hain faces mounting questions over how he bankrolled his deputy leadership bid.

He spent £185,000 but finished fifth out of six candidates.

Mr Hain initially declared just £82,000 to the electoral watchdog and in the register of interests. The full declaration was only made late on Thursday night when MPs had left Westminster for the weekend.

It revealed that £51,613 came from a firm, described as a "think tank", which was set up by his campaign treasurer.

The names of the wealthy supporters who gave money to the Progressive Policy Forum were not registered.

Mr Hain's declaration also revealed he took cash from a series of backers who he dealt with as a minister.

He accepted £10,000 from the boss of a Welsh demolition company, which he repeatedly praised as a minister, and the same amount from a drugs firm boss. The donations may have saved him having to pay campaign debts of up to £100,000 from his own pocket.

Mr Hain has said he missed declaration deadlines because he was busy with his job.

Labour MP Martin Linton, a member of the minister's campaign team, yesterday insisted there was no intention to deceive.

He said the failure to declare the donations was "no big deal" and such things "happen very easily in politics". He added: "Anyone who's worked with Peter would know it could only be what he says - a perfectly innocent oversight."

Yet left-winger Mr Cruddas said he had taken a "hardline" approach to donations to his bid and had immediately hired a lawyer and an accountant.

Tory MP David Davies, who lodged the complaint with Mr Lyon, said Mr Hain may have been unaware of campaign rules but must have known cash had to be declared in the register of MPs' interests. Mr Davies said:

"There has to come a point where people say it is not feasible for him to run two departments when all this has gone on."

Elfyn Llwyd, leader of Welsh nationalists Plaid Cymru, called the revelations "dynamite" and said Mr Hain should quit.

He went on: "More in sorrow than in anger, I am forced to say his position is untenable. This appears to be playing fast and loose with the law and ministers can't do that."

Mr Lyon will prepare a report for the powerful Standards and Privileges committee.

£200

The amount Hain was fined in 1972 for criminal conspiracy in disrupting South African sporting tours to protest against apartheid